It is well known that the efficiency of furnace/boiler systems is reduced by losses in the flue stack. These losses include losses in combustion efficiency, thermal efficiency and off-cycle efficiency. Combustion efficiency is reduced by excessive draft conditions through the flue, producing an excess oxygen condition of greater than 4%. In power boilers, maximum combustion efficiency will typically have oxygen levels of 2% to 4%. Excess draft also reduces thermal efficiency, which is the amount of energy used to accomplish a selected useful result rather than escaping through the flue. Off-cycle efficiency refers to the energy escaping through the flue stack between firing cycles.
Various attempts have been made to improve the overall efficiency of the furnace system directed toward the flue stack. These include preheaters, economizers, turbulators and various damping systems. Numerous systems attempt to improve efficiency by interrupting the exhaust gas flow up the stack in various ways. While this generally improves efficiency, the amount of restriction is limited, both practically and by regulation, since too severe a restriction will result in a dangerous furnace condition, including potentially an explosion.
One device which does restrict the flow of air and does improve efficiency to some extent is shown in U.S. Pat. No. 4,953,535, to Hagan. This device is a helical flue insert which has an adjustable pitch.
The present invention also uses a helical restrictor element, but it is configured in such a way as to produce a further significant improvement in furnace efficiency.